Development of the solid electrolyte type fuel cell having a laminated structure for which the solid electrolyte layer composed of an oxide ion conductor is sandwiched between an air electrode layer (oxidant electrode layer) and a fuel electrode layer is progressing as the fuel cell for use in the next (third) generation electric power production. In a solid electrolyte type fuel cell, oxygen (air) is supplied to an air electrode section and a fuel gas (H2, CO, and the like) is supplied to a fuel electrode section. Both the air electrode and the fuel electrode are made to be porous so that the gases may reach the interface between the air electrode and the solid electrolyte and the interface between the fuel electrode and the solid electrolyte.
The oxygen supplied to the air electrode reaches the neighborhood of the interface in contact with the solid electrolyte layer through the pores in the air electrode layer, and in that portion, the oxygen receives electrons from the air electrode to be ionized into oxide ions (O2−). The oxide ions move by diffusion in the interior of the solid electrolyte layer toward the fuel electrode. The oxide ions reach the neighborhood of the interface in contact with the fuel electrode and in that portion, the oxide ions react with the fuel gas to produce reaction products (H2O, CO2 and the like), and release electrons to the fuel electrode.
The electrode reaction when hydrogen is used as fuel is as follows:Air electrode: (½)O2+2e−→O2−Fuel electrode: H2+O2−→H2O+2e−Overall: H2+(½)O2→H2O
Because the solid electrolyte layer is the medium for migration of the oxide ions and also functions as a partition wall for preventing the direct contact of the fuel gas with air, the solid electrolyte layer has a dense structure capable of blocking gas permeation. It is required that the solid electrolyte layer has high oxide ion conductivity, and is chemically stable and strong against thermal shock under the conditions involving the oxidative atmosphere in the air electrode section and the reductive atmosphere in the fuel electrode section. As a material which can meet such requirements, generally a stabilized zirconia (YSZ) substituted with yttria is used.
On the other hand, the air electrode (cathode) layer and fuel electrode (anode) layer, both being electrodes, need to be formed of materials having high electronic conductivity. Because the air electrode material is required to be chemically stable in the oxidative atmosphere at high temperatures around 700° C., metals are unsuitable for the air electrode, and generally perovskite type oxide materials having electronic conductivity are used, specifically LaMnO3, LaCoO3 and the solid solutions in which part of the La component in these materials is replaced with Sr, Ca and the like. Moreover, the fuel electrode material is generally a metal such as Ni and Co, or a cermet such as Ni-YSZ, and Co-YSZ.
The electric discharge reaction in the so-called fuel cell proceeds irreversibly, and the larger the current is taken out from the cell to the outside, the more the irreversibility increases and the more the cell voltage falls. The maximum voltage obtainable in an actual fuel cell is the equilibrium electromotive force and the voltage falls with increasing irreversibility.
Such a voltage fall is caused by the diffusion-movement resistance of the electrons in the fuel electrode layer and the air electrode layer, in other words, by the ionization reaction rate of the oxide ion, and the potential deviation from the equilibrium potential in each of the air electrode layer and the fuel electrode layer is called overpotential (polarization). Electrons flow in the electrodes and ions flow in the electrolyte, and thus electric current flows to the outside. Hence, there will be a voltage loss (IR loss) corresponding to the product of the current flowing in the cell and the contact resistances between the solid electrolyte layer and the respective electrode layers and the electric resistances of the electrode materials themselves and solid electrolyte layer itself, etc., the IR loss increases proportional to the current taken out from the cell.
Incidentally, in a fuel cell, in order to raise the voltage to be taken out, a cell stack is used in which a number of electric power generation cells (single cells) are jointed in series with interconnectors; however, in this case the electric resistance of the interconnectors themselves also causes the IR loss.
All the energy corresponding to the difference between the potential converted from the thermal energy obtained by combustion of the fuel and the potential which can be taken out from an electric power generation cell is vainly released as thermal energy from the electric power generation cell of a fuel cell. Therefore, the overpotential of each of the above described electrodes and the amount of the IR loss inside each electric power generation cell will influence the electric power generation efficiency of a fuel cell greatly.
FIG. 2 shows the relation between the voltage and current density which can be taken out from a single cell. As shown in the figure, the voltage which can be taken out from a single cell falls from the equilibrium electromotive force of the cell by the voltage corresponding to the sum of the absolute overpotential values and the IR losses in the fuel electrode layer and the air electrode layer; as aforementioned, the fall of the voltage becomes large with increasing current density.
The overpotential is especially large in the fuel electrode in a solid oxide type fuel cell which uses for the electrolyte an oxide ion conductor mainly composed of a lanthanum gallate based oxide; the fuel electrode overpotential in current fuel cells is generally of the order of 100 to 150 mV, presumably exceeding 150 mV as the case may be. If the working voltage of an electric power generation cell is set to be of the order of 0.7 V (corresponding to the current density of about 2 A/cm2 in FIG. 2), the effect of the above described overpotential value of 100 to 150 mV on the voltage of 0.7 V is very large, and room is still left for improvement of the countermeasure against the electrode overpotential.
Especially the IR loss is largely affected by the electric resistance of the solid electrolyte layer and the contact resistance thereof with the electrodes, and accordingly a number of studies have hitherto been made on the materials and the layer thickness thereof.